2,14-16 However, this near-zero temperature coefficient can increase when crystals are immersed in liquid. The effects of temperature on frequency for various angle cuts are well known and documented. The common 5 MHz quartz crystal has a corresponding thickness of approximately 330 µm.ĪT-cut crystals are designed to have good stability at room temperature (25 ☌), i.e., minimum frequency change (1-3 Hz/☌). The resonance frequencies are typically on the order of MHz and inversely proportional to the crystal thickness. When the corresponding alternating current is applied to the quartz disc, it will oscillate at its resonance frequency. 13 After cutting, a pair of metal electrodes (usually gold) is directly evaporated on the top and bottom surfaces of the quartz disc (Figure 1). The quartz discs used in QCMs are most commonly processed using the “AT cut” that provides pure thickness shear mode oscillation where the two surfaces of the crystal move in an anti-parallel fashion. To fabricate quartz crystal resonators, wafers are cut from the bulk quartz crystal at specific orientations with respect to the crystallographic axis. In addition, α-quartz, the phase of quartz that can be used as a resonator, is thermodynamically stable up to 573 ☌. It is found in abundance in nature, and it is easy to grow and process. Quartz, besides being piezoelectric, also possesses a unique combination of properties that make it an ideal candidate for ultrasensitive devices. 11 A year later they demonstrated that the converse effect was also possible, i.e., the quartz could deform upon the application of voltage. The French physicists Pierre and Jacques Curie discovered this effect in 1880 when they demonstrated that salt crystals could produce electricity when deformed along certain crystallographic orientations. Piezoelectricity is defined as the generation of electricity in response to the mechanical deformation caused by mechanical stress or as the generation of physical deformation on the application of electricity in such crystals. The word piezoelectricity is derived from the Greek word “piezein”, which means “to press”, and the electricity that is generated in response to applied pressure in these types of materials. The operation of QCM is based on the piezoelectric effect that occurs in crystalline materials of certain crystallography known as “acentric” materials, 11-12 Quartz belongs to this class of crystals. 5-10 Theory and Instrumentation Quartz: A piezoelectric resonator Some examples include: kinetics of molecular interactions (e.g., protein-protein interactions), molecule-surface interactions (e.g., affinity of bio-molecules to the binding site of a functionalized surface), buildup of polymer films and their interactions with different constituents of liquid media, effect of surfactants on various coatings, biosensor applications, etc. QCM-D has a wide range of applications in various fields of science. The maximum film thickness that can be measured varies from several hundred nanometers to a few microns, depending on the rigidity of the film. The maximum mass sensitivity of the technique in liquid is less than 1 ng/cm 2. QCM-D also provides real-time information on the viscoelastic properties of the adsorbed film, such as viscosity, elasticity, and density. This technique is called Quartz Crystal Microbalance with Dissipation (QCM-D). Furthermore, by monitoring both Δf and ΔD, additional structural information (e.g., conformational changes in the film, crosslinking, and swelling) can be obtained. Both frequency change (Δf) and energy loss (measured as dissipation change, ΔD) need to be measured to accurately determine the mass change of a viscoelastic film. The mass of such films cannot be determined accurately by measuring the frequency change alone. This can lead to dampening, or energy loss, of the oscillation. Adsorption can produce soft, or viscoelastic films and the resulting layer may not fully couple to the oscillating crystal. 1-2 In the liquid environment, however, molecular adsorption includes contributions from associated liquid molecules as an additional dynamic mass via direct hydration/solvation and/or entrapment within the adsorbed film. The QCM can provide useful information on the amount of mass deposited and the rate of deposition (or removal) of such films by monitoring the real-time change in frequency. Hence, the change in mass of such films is linearly related to the change in the oscillation frequency which is defined by a well-known equation called the Sauerbrey Equation. Molecular adsorption via the vacuum or gas phase typically results in rigid films that are fully coupled to the oscillation of the electrode surface. Electrochemical Quartz Crystal Microbalance
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